1. Technical Field
The present invention relates to a method for manufacturing an organic electroluminescent device and an organic electroluminescent device manufactured by the method.
2. Related Art
There has been developed, for example, electro-optic devices such as an organic EL display, optical writing heads such as a line head, and image-forming devices such as an optical printer, using organic electroluminescent (hereinafter referred to as organic EL) elements, which are a type of electro-optic element. An organic EL element generally has a structure including a luminescent layer made of an organic material between a pair of opposing electrodes. An organic EL device including such organic EL elements can display full color images by using red (R), green (G) and blue (B) color filters, or by using respective organic EL elements emitting those color lights.
The organic EL element is formed by different methods depending on whether the luminescent material (hereinafter referred to as organic EL material) of the organic EL element has a high molecular weight or a low molecular weight. For high-molecular-weight organic EL materials, it has been known that a so-called liquid ejection method can be applied. More specifically, an organic EL material is dissolved or dispersed in a predetermined solvent to prepare a liquid composition. The liquid composition is applied onto predetermined pixel electrodes by the liquid ejection method, thus forming organic EL elements.
The liquid ejection method allows a highly controlled minute amount of liquid composition to be applied to desired positions. Thus, the process using the liquid ejection method can advantageously form highly precise patters of organic EL materials. In addition, it is proposed for the liquid ejection method that a partition wall (bank) is formed around each pixel electrode onto which the organic EL material will be applied. Thus, the pixels are separated from each other. According to, for example, Japanese Unexamined Patent Application Publication Nos. 2002-334782 and 2002-372921 and Japanese Patent No. 3036436, the presence of the bank can prevent the liquid composition of the organic EL material applied onto a pixel electrode from mixing with the liquid composition applied onto other pixel electrodes, thus ensuring reliable patterning.
In order to reduce the current flowing to the organic EL element or in order to increase the brightness, many of the recent organic EL devices have a structure in which a plurality of organic EL elements are surrounded by a common partition wall taller than the bank so as to be divided into groups. This structure can also be formed by the liquid ejection method. More specifically, a liquid is applied to a region or group including a plurality of organic EL elements surrounded by a common partition wall by a liquid ejection method, followed by drying. Thus, the same layer is formed of the same organic EL material for the plurality of organic EL elements surrounded by the common partition wall. However, such a process causes a new problem. The applied liquid flows and moves during drying, and results in thickness variations.
The liquid ejection method for applying a liquid to a plurality of organic EL elements surrounded by a common partition wall may cause the section surrounded by the common partition wall to have a difference in dried state between the outer region, that is, the region close to the common partition wall, and the inner region. This is because the partial pressure of the solvent vapor from the liquid composition, that is, the solvent vapor concentration, around the common partition wall is lower than that in the inner region of the section surrounded by the common partition wall. Consequently, the solvent can be more rapidly dried from the outer region than the inner region. The difference in evaporation rate of the solvent causes a difference in volume between the rapidly dried region and the slowly dried region even though the liquid composition is applied to the same section in the same manner. Thus, the composition flows from the region where the solvent is not dried much to the region where the solvent is dried to reduce the volume. Consequently, the resulting layer has an uneven thickness (thickness variations). The thickness variations varies the amount of light emission, and thus leads to variations in brightness or color tone and results in display failure (inconsistencies in displayed images).
In addition, the thickness in the vicinity of the common partition wall depends on whether the common partition wall is liquid affinity or liquid repellency to the liquid composition. For example, a liquid composition close to a liquid affinity partition wall can spread over the surface of the partition wall even during drying. Accordingly, the liquid composition in the vicinity of the common partition wall rises along the side surface of the common partition wall to increase the thickness. Thus, thickness variations occur. In order to solve the above problems, Japanese Unexamined Patent Application Publication No. 2007-87693 has proposed that organic EL elements are formed apart from the common partition wall by a predetermined distance to prevent thickness variations.
The stereoscopic shape (relief shape) of the base layer underlying the region where the liquid composition is applied significantly affects the drying conditions that may cause thickness variations. Since, for example, the liquid composition is easily collected in recesses, the amount of the solvent vaporized from the liquid composition in the recesses is different from that from the composition on flat regions. When the liquid composition flows and moves during drying, how easy the liquid composition is to flow depends on the stereoscopic shape of the base layer. Consequently, the resulting layer has an uneven thickness. In addition, the flow direction of the liquid composition depends on various factors, such as the liquid affinity of the common partition wall and the volatility and evaporation drying rate of the solvent in the liquid composition, being not fixed in a specific direction. Also, it depends on the stereoscopic shape. It is therefore difficult to reduce thickness variations resulting from the flow of the liquid composition.
In the above-cited patent document, the films of the liquid composition are formed apart from the common partition wall by a specific distance or more, and the films around the partition wall are not used to eliminate the negative effect of thickness variations. However, the patent document does not propose any solution of the problem resulting from the stereoscopic shape of the base layer. Furthermore, the region distant from the common partition wall is flat, and accordingly liquid composition applied to such a flat region is liable to flow. If the distance from the common partition wall is increased to eliminate the negative effect of thickness variation in the vicinity of the common partition wall, the liquid composition becomes liable to flow and the risk of thickness variations is increased on the contrary.